If you got a brand-spanking-new Surface Pro 4 Type Cover, especially the one with the fingerprint reader, or a Surface Dock, and you wanted to play with them on your Surface Pro 3, make sure you visit good ol’ Windows Update and make sure you have the 10/26/2015 System Firmware Update. Otherwise you will experience very limited functionality.
The fingerprint reader won’t be recognized, therefore you won’t even see the Microsoft Hello section of the Sign-on Options screen. The dock will charge your Surface, but the Surface won’t see the DisplayPort or USB ports.
See the whole scoop from Microsoft here: http://www.microsoft.com/surface/en-us/support/install-update-activate/pro-3-update-history?os=windows-10